Printers have many types, such as inkjet printers, laser printers, heat transfer printers, sublimation printers and the like. However, regardless of what types of printers, if intending to have a broader application scope and finer printing quality, all printers are additionally equipped with automatic detection functions of ink of printing ribbon and length of printing papers. For example, a circuit of an opto-interrupter mounted to a thermal printer is one of the typical examples. The circuit is adopted to control voltage or vary load impedance to set up the dynamic ranges of a transmitter and a receiver so as to increase circuit recognition in response to different operation requirements.
Whereas, the transmitter of the conventional structural design normally employs regular light-emitting diode (LED), and the load impedance adopted by both of the transmitter and the receiver is usually fixed. Accordingly, the adopted dynamic range is limited and poor. Despite digital potentiometers currently employed by some advanced designs to accurately adjust the input and output load impedance values, the major drawback lies in that the integrated circuits of those designs have relatively higher costs.
With reference to FIG. 1, the inventor of the present invention had implemented some changes on a power adjusting structure of an opto-interrupter, namely, a plurality of load impedances ZI and ZL respectively mounted to two sides of the opto-interrupter to correspond to m and n IC control pins, to have variation of mth power of 2 and nth power of 2 combinations. As a result, the dynamic ranges of the transmitter and the receiver can be respectively adjusted to simultaneously realize multi-stage adjustment and lower structure cost.
Such design has a broader operating range and better penetration of printing medias (including label paper and printing ribbon) than conventional structures. However, current printing medias are of varieties and different in quality, and their light transmittance falls in pretty wide dynamic range. The light transmittance of some media may be good, but that of some others may be poor. If the adjustable two-sided load impedance circuit structure is the only one adopted, the wide dynamic range for different printing media can be hardly covered. When encountering special or thicker printing media, such circuit structure still fails to deliver effective light transmittance (as shown in FIG. 2).